1. Field of the Invention
The present invention relates to an injection molding machine, and more particularly, to an injection unit of an injection molding machine provided with a load detection unit for detecting resin pressure on the injection screw.
2. Description of the Related Art
In an injection unit of an injection molding machine, a screw-type injection unit (that is, a screw-in-line-type injection unit) melts and kneads resin inside a heating cylinder by rotating an injection screw, and further, retracts the injection screw while applying pressure on the resin with the injection screw and measures the melted resin at that retracted position. Thereafter, the injection unit injects melted resin into a mold by advancing the screw.
As a result, it is necessary that the injection unit be provided with a mechanism that rotates the injection screw and a mechanism for driving the injection screw in an axial direction and injecting the resin.
An injection mechanism is known that comprises an injection unit by providing a pusher plate to which is fitted an injection screw so as to be freely rotatable but axially unmovable, providing on such pusher plate a pulley that rotates the injection screw and a screw rotation motor that drives the pulley by a belt, and further, providing thrust force drive means for injecting the resin with the injection screw by driving such pusher plate in the injection screw axial direction, and further, has a load detection unit such as a load cell for detecting the pressure on the injection screw (for example, JP 2-16023A and JP 9-174628A).
A load detection unit such as a load cell detects the resin pressure inside the heating cylinder, and the resin pressure detected by the load detection unit is used in back pressure control during a measuring step. In addition, in injection and pressure holding steps, the detected resin pressure is used in injection pressure feedback control, pressure holding feedback control and the like. Hence, it is desirable that the resin pressure be detected with greater accuracy.
As the means for rotating the screw, in that which uses a pulley-and-belt transmission mechanism, the belt is run between a drive pulley provided on the motor and a driven pulley provided on the injection screw side. This belt exerts a force in a radial direction on the pulley provided on the injection screw side, creating moment on the shaft on which the pulley is mounted, causing the force of friction of the injection mechanism unit to fluctuate and affecting the resin pressure detected by the load cell. Inventions that prevent these things from happening are also known (for example, JP 2000-117789A and JP 2000-334789A).
In an injection unit in which, as the means for rotating the injection screw mounted so as to be freely rotatable and axially unmovable on the pusher plate, the screw rotation motor is mounted on the pusher plate and a belt such as a timing belt is run between the drive pulley mounted on the output shaft of the motor and the driven pulley mounted on the injection screw side for driving the injection screw, and such belt transmission mechanism is used to rotate the injection screw, the force exerted by the belt on the drive pulley in the radial direction of the drive pulley also acts on the pusher plate through the motor that is mounted on the pusher plate, adversely affecting resin pressure detection of the load cell or other such load detection unit provided on that pusher plate.